The use of 3,5-dibromo-4-hydroxybenzonitrile as an herbicide is well established (R. L. Wain Nature, 1963, 200, 28; K. Carpenter and B. J. Heywood Nature, 1963, 200, 28-29). Certain esters of 3,5-dibromo-4-hydroxybenzonitrile, especially the octanoate, have been used extensively as herbicides in the control of broad-leafed weeds, particularly in crop growing areas. Although the esters have thus been shown to be highly effective, they have heretofore been produced only by incurring productivity penalties, which are aggravated by the expensive and cumbersome purifications that have heretofore been required.
Auwers and Reis (1896) in a paper Chem. Ber., 1896, 29, 2355-2360 prepared 3,5-Dibromo-4-hydroxybenzonitrile by a complex four step procedure starting with 4-hydroxybenzaldehyde, an uneconomical procedure involving bromination, formation of the aldoxime, dehydration with concomitant acetate formation and hydrolytic removal of the acetate. The drawback of the process is multi steps are involved to get the final product which makes the process uneconomical.
E. Muller, et al. (1959), [Chem. Ben 1959, 92, 2278], has described a procedure for bromination of 4-hydroxybenzonitrile with elemental bromine in methanolic acetic acid. The product produced by the process was recovered by contacting the resulting bromination mixture with aqueous methanolic sodium hydrogen sulfite. The drawback of the process is that elemental bromine is used which is hazardous and difficult to handle.
U.S. Pat. No. 3,349,111 assigned to R. W. Luckenbaugh et al. describes the production of 3,5-dibromo-4-hydroxybenzonitrile or its sodium salt by carrying out bromination with elemental bromine in an aqueous suspension, especially aqueous caustic, followed by a chlorine spurge. The drawback of the process is that elemental bromine is used which is hazardous and difficult to handle. Besides, being an exothermic reaction it is required to cool externally to maintain the desired temperature of the reaction mixture.
U.S. Pat. No. 4,349,488 assigned to D. A Dentel, et al. describes the preparation of 3,5-dibromo-4-hydroxybenzonitrile with liquid bromine using chlorobenzene as solvent under reflux conditions at a temperature about 134° C. The liquid bromine was charged in the reactor for a period for 1 to 1.5 hours. The drawback of the process is that elemental bromine is used which is hazardous and difficult to handle. The temperature was above 130° C. and chlorobenzene was used as solvent is carcinogenic.
U.S. Pat. No. 4,436,665 assigned to R. E Sheds, describes the preparation of 3,5-dibromo-4-hydroxybenzonitrile either with liquid bromine or bromine and chlorine either sequentially or at the same time or with pre-formed bromine chloride or 3% hydrobromic acid. The drawback of the process is the use of elemental bromine which is hazardous and difficult to handle and special equipments are required.
French Pat. No. 1,375,311 describes bromination of hydroxybenzonitrile in acetic acid, obtaining 3,5-dibromo-4-hydroxybenzonitrile in 60% yield. The patent also suggests that by bromination with aqueous sodium hypobromite the product may be obtained in 78% yield. The drawback of the process is that the yield was in the range of 60 to 78% and use of hazardous liquid bromine in acetic acid.
Ramachandraiah et. al, U.S. Pat. No. 6,740,253 describes the process for the preparation of non-hazardous brominating reagents making use of alkaline intermediate from bromine recovery process and alkaline/chlorine at ambient temperature. In this brominating reagent the bromide to bromate ratio was in the range of 2:1 to 2.2:1.0. The limit of the cited patent was only to prepare the brominating reagent and does not to reveal about the preparation of 3,5-dibromo-4-hydroxybenzonitrile.
Ramachandraiah et. al, U.S. patent Ser. No. 10/449,723, describes an improved process of the preparation of brominating reagent by in situ generation of oxidizing agent by purging chlorine to the alkaline bromine solution at ambient temperature. Here also the scope of the invention was limited to the preparation aspects of brominating reagent and does not reveal about the preparation of 3,5-dibromo-4-hydroxybenzonitrile.
Bedekar et. al, U.S. Pat. No. 6,956,142, describes an eco-friendly process for the preparation of bromo benzene using benzene and the brominating reagent at reflux temperature of benzene. Here also the scope of the invention was limited to the preparation of bromobenzene and not mentioned about the preparation of 3,5-dibromo-4-hydroxybenzonitrile.
Varshney, et al. Indian Patent No. 180996 has described an improved process for the synthesis of 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) from p-cresol comprising (i) brominating p-cresol to make 3,5-dibromo-p-cresol, (ii)oxidizing 3,5-dibromo-p-cresol to 3,5-dibromo-4-hydroxybenzaldehyde; (iii) oximating 3,5-dibromo-4-hydroxybenzaldehyde to 3,5-dibromo-4-hydroxybenzyl oxime and (iv) dehydrating 3,5-dibromo-4-hydroxybenzyl oxime to 3,5-dibromo-4-hydroxybenzonitrile or bromoxynil. The drawback of the invention is that it involves multi steps which make the process cost sensitive and the various steps makes the process complicated.
Vidyasagar, et al. in a paper published in Indian Journal of Chemistry, Section B: (1993), 32B, 872 described a two-step synthesis of 3,5-dibromo-4-hydroxybenzonitrile from p-cresol. In the first step p-cresol was brominated with liquid bromine and in the second step it was treated with nitro ethane/fused NaOAc in AcOH to give bromoxynil. The drawback of the process is liquid bromine is used as a source of bromine which is hazardous and difficult to handle. Moreover, multi steps are involved and the yield is 87%.
The prior art does not divulge nor teach how 3,5 dibromo-4-hydroxybenzonitrile can be prepared making use of inorganic salts and mineral acid. It is reported for the first time in the present invention how product could be obtained in absence of hazardous liquid bromine, without organic solvent and catalyst at under ambient conditions.
The inventive steps involved in the present invention are i) the soluble brominating reagent with active bromine dispenses the need of hazardous liquid bromine, (ii) the reaction moves forward towards completion without the need of any catalyst, (iii) the process uses only water as dispersing medium and alleviates the need of any solvent and catalyst, v) maximum bromide atom efficiency, and (iv) the reaction is completed at ambient temperature.